auxillary memory organization by sanjiv nambiar

7/27/2019 Auxillary Memory Organization by Sanjiv Nambiar

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Auxiliary Memory Organization


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The main memory construction is costly. Therefore, it has to be limited in size.

The main memory is used to store only those instructions

and data which are to be used immediately.

However, a computer has to store a large amount of information.

The bulk of information is stored in the

auxiliary memory.


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This is also called

backing storageorsecondary storage.They include hard disk, floppy disks, CD-ROM, USB flash drives, etc.


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When the electricity supply to the computer is off, all

data stored in the primary storage is destroyed.

On the other hand, this is not true forsecondary storage.

The data stored in secondary storage can be stored for

the desired time


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Auxiliary Memory Devices

Magnetic media Tape

Disks

Optical Media Compact Discs

CD-R, WORM (Write

Once, Read Many)

CD-RW DVD

DVD-R

DVD-RW


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The important characteristics of any

device are its

access mode,

access time,transfer rate,

capacity,

&

cost.


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Access ModeThere are Many Types of Storage Devices those are based on the

Sequential and Random Access Means the data which is Storedinto the Secondary Storage devices can be

Read either from the First Location which is also known as the

Sequential Access or Sequential Manner and

the Data can be Read from these Disks and also from any

Locations. So if any Disk provides this Utility then this is called as

the Direct Access Mechanism.

There are Many Storage Devices those are either based on the

SASD or Some are DASD.


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The average time required to reach a storage location

in memory and obtain its contents is called

the access time.

The transfer rateis the number of characters or words that the device

can transfer per second, after it has been positioned at

the beginning of the record

CapacityThe amount of data it is possible to store on a

medium

CostHow much it costs per megabyte


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Magnetic Media


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Magnetic Tape

Magnetic tape is amedium for magneticrecording generallyconsisting of a thinmagnetically coating on along and narrow strip ofplastic. Nearly allrecording tape is of thistype, whether used forrecording audio or video

or for computer datastorage.


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Devices that record and playback audio and video usingmagnetic tape are generally called tape recorders and videotape recorders respectively. A device that stores computer dataon magnetic tape can be called a tape drive, a tape unit, or astreamer.


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Pros and Cons of Magnetic Tape

Inexpensive to store large amounts of

information.

A reel of tape could store as much as 225

MB in the early 1980s.

No other storage technology could

compare to the price of tape

Sequential access Slow access

Tapes are highly susceptible to magnetic fields

and changes in temperature and humidity

Tapes are also susceptible to physical damage

Tape miss-feeds

Magnetic particle instabilities

Substrate deformation


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Magnetic Tape Uses Today

Today, tape based systems are

usually used for backup purposesonly.


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Hard Disks Today, most people use Hard Disks for

secondary storage

The basic technology used in hard disks is

similar to that of magnetic tape

Magnetic material is layered onto ahigh-precision aluminum disk

The disk head can move to any point

on the platter almost instantly

compared to tape

Tape moves at approx 5 cm/s. Disk

platters move at up to 7500 cm/s (272

km/h!)


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Hard Disks

To increase capacity, a hard disk will usually contain

several platters


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The heads never touch the platters, but they are very

close. This makes hard disks susceptible to mechanical

shock.

Hard Disks


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Storing Information on a Hard Disk Each platter is broken up into tracks and sectors

Tracks are concentric circles on the disk

Each track is broken up into a series of sectors

Track (yellow ring)

Sector (yellow ring

between the lines)


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Sectors and Blocks Sectors are further broken up into blocks

A block is a fixed size unit of storage

512 bytes/block is most commonly used

1024 bytes/block is common with SCSI disks

2048 bytes/block is used with CDs

If the user stores onto the hard disk a file which is larger

than the block size, then multiple blocks are used.


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Optical Media


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Optical Media Optical disks are very much like hard disks

Hard disks store information using magnetic material

Bits are stored by changing the magnetic properties

of the magnetic material

Bits are read by picking up the tiny magnetic fieldwith a read head

Optical disks store information as pits in a physical

medium

A laser is used to determine if a pit is present or not.


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CD-R CD Roms must be pressed. They are read only

CDR discs can be written once and read many times

CDRs are made out of aluminum and plastic, but also

contain a dye layer

This dye is modified by a laser when the disc is being

written The laser heats up the dye and it becomes non-reflective

Label

Acrylic

Aluminum

Plastic

Dye


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Storing Information on a CD Because CDs were originally intended for audio output,

there is a single track of data which spirals out from thecenter of the disc


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CD-RW CD-RW is similar to CD-R

The main difference is that the dye can be made reflectiveagain through an erase process

In this way, CD-RW discs can be written many times

Too much erasing, and the dye starts to fade.

Label

Acrylic

Aluminum

Plastic

Dye


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DVD Digital Versatile Disk DVDs hold approx 7 times the information that CDs do in

the same amount of storage space

DVDs come in 3 types

Single Sided/Single Layer (4.7 GB)

Single Sided/Double Layer (8.5 GB)

Double Sided/Double Layer (17 GB)

DVD uses a laser with a shorter wavelength so the pits are

smaller

More pits can be stored on a DVD

Narrower track


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Thankz !

auxillary memory organization by sanjiv nambiar

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